Glass is desirable for numerous properties and applications, including optical clarity and overall visual appearance. For some example applications, certain optical properties (e.g., light transmission, reflection and/or absorption) are desired to be optimized. For example, in certain example instances, reduction of light reflection from the surface of a glass substrate may be desirable for storefront windows, display cases, photovoltaic devices (e.g., solar cells), picture frames, other types of windows, greenhouses, and so forth.
Photovoltaic devices such as solar cells (and modules therefor) are known in the art. Glass is an integral part of most common commercial photovoltaic modules, including both crystalline and thin film types. A solar cell/module may include, for example, a photoelectric transfer film made up of one or more layers located between a pair of substrates. One or more of the substrates may be of glass, and the photoelectric transfer film (typically semiconductor) is for converting solar energy to electricity. Example solar cells are disclosed in U.S. Pat. Nos. 4,510,344, 4,806,436, 6,506,622, and 5,977,477, the disclosures of which are hereby incorporated herein by reference.
Substrate(s) in a solar cell/module are sometimes made of glass. Incoming radiation passes through the incident glass substrate of the solar cell before reaching the active layer(s) (e.g., photoelectric transfer film such as a semiconductor) of the solar cell. Radiation that is reflected by the incident glass substrate does not make its way into the active layer(s) of the solar cell, thereby resulting in a less efficient solar cell. In other words, it would be desirable to decrease the amount of radiation that is reflected by the incident substrate, thereby increasing the amount of radiation that makes its way to the active layer(s) of the solar cell. In particular, the power output of a solar cell or photovoltaic (PV) module may be dependant upon the amount of light, or number of photons, within a specific range of the solar spectrum that pass through the incident glass substrate and reach the photovoltaic semiconductor.
Because the power output of the module may depend upon the amount of light within the solar spectrum that passes through the glass and reaches the PV semiconductor, certain attempts have been made in an attempt to boost overall solar transmission through the glass used in PV modules. One attempt is the use of iron-free or “clear” glass, which may increase the amount of solar light transmission when compared to regular float glass, through absorption minimization.
Another attempt to boost overall solar transmission involves the use of porous silica as an antireflective coating on glass substrate. But the environmental durability of AR coatings derived from porous silica may be an issue if the coating is cast on the glass substrate at high humidity and/or temperature. When water contacts glass, an ion exchange process may begin, in which sodium ions in the glass are displaced by hydrogen ions from the water. The immediate outcome can be the hydration, or dealkalization, of the glass and depletion of the hydrogen ions from the water. This process can be accompanied by a shift in the aqueous equilibrium to produce more H+ and OH− ions (i.e., H2O→H++OH−).
This ion exchange process may be temperature and humidity dependent. If this process occurs over a sufficiently long period of time, there may be degradation in the surface quality due to alkali attack on the glass silicate network. This degradation may manifest itself in one or more forms, such as: (1) A distinctive milky white haze, which may be seen in all the glass (with or without a coating) after reaction in high humidity and/or freezing conditions; and/or (2) Microscopic pitting of glass occurs, wherein the pits may develop into tiny crevices that grow and eventually undercut the surface, forming islands of glass which can exfoliate from the underlying bulk material.
These defects may lead to a reduction in transmissivity of an antireflecting coating after the high humidity and temperature variation. Therefore, there may be a need to minimize the reduction in transmission to maintain the performance of the antireflecting coatings in the environmental conditions such as high humidity and temperature conditions.
In one aspect of the present invention, there is a capping layer on antireflecting coatings that may minimizes the direct contact of water to the coating and substrate. It may lead to an environmentally durable AR coating. Accordingly, in one embodiment, this invention relates to use of a capping layer, such as, an antifog coating on a temperable AR coating on glass substrate, and possibly a minimization of reduction in transmittance after the exposure of high humidity and temperature conditions (such as, for example, thermal and dampness/wetness testing).